Chemistry
Chemistry 14: 1-2
WebLecture
Reactions Rates
Outline
Reaction Rates
Reactions occur in real time, so we need to understand how fast they occur, and what factors can change this rate. For a reaction that occurs in solution, we can define the rate of a reaction as the change in concentration of one of the reaction components over time:
Rate = Δ[A]/Δt
Here, A could be a reactant or a product species.
If we have several components in a reaction like A + C → B + D, we may have four different rates of change: Δ[A], Δ[B], Δ[C], or Δ[D]. These will vary to some extent (but not entirely) depending on the stoichiometric coefficient of the species in the balanced reaction. In order to completely understand what it happening in a particular reaction, we need to know the rate at which each component is changing.
If concentration is the only factor, then the molarity of reactants and products will change as the stoichiometric coefficients change. For example, suppose that we have 1 mole of reactant A which dissolves to form 2 moles of product B in a 1L solution.
A → 2B
The rate of change of the concentration of A (symbolized [A]) will drop half as fast as the increase the concentration of B (symbolized by [B]). We would have the ratio
- Δ[A]/Δt = ½Δ[B]/Δt
Note the negative sign on the rate of change of concentration for A: it is decreasing, while [B] is increasing.
Factors other than the ratio of moles in the balanced equation may affect the actual observed reaction rate. To determine a reaction rate for a specific reaction, we use a pH indicator, a chronograph, or a mass spectrometer to measure concentration at specific intervals. Then we plot the concentration values over time. In most situations, one of three possible reaction patterns will show up for the concentration change of reactant in solution:
| The reaction rate is constant. Concentration changes at a steady rate, regardless of what the concentration level is. A high concentration or low concentration doesn't matter. The rate is independent of the concentration. The reaction rate is constant. |
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| The reaction rate is high while the concentration rate is high, but as the reactant is consumed by the reaction, the rate falls steadily as the concentration drops. Here the rate is directly dependent on the concentration. The rate at which the reaction rate changes is dependent on the concentration, which is changing.. |
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| The reaction rate is high but falls quickly as the square (or a higher power) of the concentration. The rate at which the reaction rate changes is dependent on a power of the concentration, which is changing. |
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Practice with the Concepts
Concentrations
- What factors might affect a reaction rate, besides concentration?
Optional Readings
Need more drill work on significant figures? Scientific notation? Concepts of mass, density, and volume? Percents? ...You name it, you can probably work on it at the Chemistry drill site, courtesy Scott Van Bramer at Widener University.
Check out the UC Davis CHEMWiki page on factors affect rate laws for more discussion on how concentration, temperature, solvent, phase, surface area, and catalysts can change the rate of a reaction.
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